The present invention relates to switches for electrical circuits. In a particularly preferred embodiment, the present invention relates to switches that are used in industrial applications, such as limit switches.
Switches are commonly employed as input devices to indicate the presence or absence of a particular condition in a system or process that is being monitored and/or controlled. Switches have found particular use in industrial control systems, where it is often desirable to monitor conditions so that appropriate actions may be taken in response to the monitored conditions. For example, limit switches are typically utilized in industrial control applications to automatically monitor and indicate whether the travel limits of a particular device have been exceeded.
Certain characteristics have been found to be desirable in switches. First, it is desirable in many applications to provide a switch with a short trip point. The trip point refers to the amount of movement that must be incurred by an actuation mechanism of the switch before the switch trips. A short trip point is desirable, because a short trip point implies that the actuation mechanism must only travel a short distance and therefore that the limit switch responds more quickly to an external event.
Second, it is desirable in many applications to provide a switch that changes between an unactuated state and an actuated state in as little time as possible. This allows the switch to change states and provide a stable output respond more quickly after the occurrence of the external event.
Third, it is desirable in many applications to provide a switch that is capable of being manually forced to break a weld in the event that the contacts become welded to a spanner or other components of the switch assembly. In some switch applications, the contacts and the spanner of the switch may unintentionally and undesirably become welded together, for example, due to a transient voltage or current in-rush condition. In these situations, it is desirable to make it possible to forcibly break the weld if enough force and displacement are applied to an actuator mechanism of the switch. It is further desirable, for sake of ruggedness and durability, to provide a mechanism that is capable of breaking a weld using a mechanism that consists of only of rigid body components. Indeed, ruggedness and durability are themselves additional desirable characteristics of switches.
While switches that to some extent have one or more of these characteristics have previously been provided, further improvements are still needed. A switch and a method of operating a switch that offers an improvement over existing designs in one or more of the above-mentioned aspects would be highly desirable.
According to a first preferred embodiment of the invention, a switch comprises first and second contacts, a plunger, a toggle bar, a spanner and a trip lever. The plunger is capable of receiving an actuating force and is moveable in response to said actuating force. The toggle bar is also moveable and has first and second positions. The spanner has first and second positions that respectively correspond to the first and second positions of the toggle bar. The spanner establishes an electrical connection between the first and second contacts when the spanner is in one of the first and second positions, and does not establish the electrical connection between the first and second contacts when the spanner is in the other of the first and second positions. The trip lever is operatively disposed between the plunger and the toggle bar. The trip lever is capable of receiving the actuating force from the travel plunger at an input location on the trip lever and transferring the actuating force to the toggle bar at an output location on the trip lever, causing the toggle bar to move from the first position to the second position of the toggle bar. The output location of the trip lever travels a greater distance than the input location when the actuating force is transferred to the toggle bar.
According to a second preferred embodiment of the invention, a switch comprises a pretravel plunger, first and second levers, an over-travel plunger, first and second contacts, and a spanner. The pretravel plunger is moveable in a longitudinal direction in response to an actuating force, and includes a post that is moveable with the pretravel plunger in the longitudinal direction. The first and second levers are hingedly mounted and are moveable in response to movement of the post to permit an aperture to form between the first and second levers that is sufficient in size to permit the post to pass between the first and second levers. The over-travel plunger is moveable along the longitudinal axis in response to movement of the first and second levers. The over-travel plunger has a cavity formed therein at a longitudinal end adjacent the post. The cavity is capable of receiving the post after the post passes between the first and second levers. The spanner is moveable between first and second positions in response to movement of the over-travel plunger. The spanner establishes an electrical connection between the first and second contacts when the spanner is in one of the first and second positions, and does not establish the electrical connection between the first and second contacts when the spanner is in the other of the first and second positions.
Other objects, features, and advantages of the present invention will become apparent to those skilled in the art from the following detailed description and accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many modifications and changes within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.